1. Field of the Invention
This invention relates to a method of producing a compound oxide superconducting thin film, and more particularly to a method of producing a superconducting thin film of a compound oxide containing copper, bismuth, vanadium or other such element having a redox charge.
2. Prior Art Statement
Following the discovery by J. G. Bednorz and K. A. Muller of the original oxide superconductor consisting primarily of copper, a number of other oxide superconductors have been developed which use the incorporation and combination of various metal oxides to increase the critical temperature at which superconductivity is exhibited. However, while numerous attempts have been made to develop new superconductors, the lack of any theoretical guidelines for increasing the superconducting critical temperature means that researchers engaged in such development have to rely on the experience they accrue through their work.
The oxide superconductors discovered up to now include among their constituent metal elements at least one element having a redox charge. Moreover, it is known that the element or elements are simultaneously present in different charge states at different portions of one and the same superconductor and that where these different charge states are regulated by oxygen, the oxygen atoms are usually not present in an integer ratio to the metal atoms. It is therefore thought that this presence of oxygen atoms at a non-integer ratio, or the variation in valence number of metal element atoms, has an effect on the characteristics of the superconductor and its critical temperature.
In the case of a superconductor consisting primarily of a copper oxide, methods that have been used to increase the critical temperature at which superconductivity is exhibited include raising the average valence number of the copper by using a flow of gas having an oxygen content that is higher than the atmospheric oxygen content or using a flow of oxygen gas itself, and increasing the ratio of copper (III) in oxygen gas under high pressure.
Superconductors have also been produced in which a substance having a redox charge such as oxides of bismuth (V), rare earth metals, alkaline earth metals and the like have been added to the copper in certain amounts to increase the valence number of the copper.
Thus, such superconductors have been produced by starting with a combination of two or more oxides. However, when the superconductors are formed as thin films for electronic device applications, there have been a number of problems, such as that it is difficult to achieve continuity of superconducting portions, so superconductivity is not readily exhibited.
With respect to most of the high-temperature superconductors that have been previously developed, carrier injection is considered to be an important factor that determines the critical temperature T.sub.c at which superconductivity is exhibited. A carrier injection method disclosed in U.S. Pat. No. 5,286,711 by the present inventors comprised producing an oxide superconductor by an electrochemical oxidation-reduction (redox) method in which the electrolytic potential for the electrochemical processing is determined on the basis of a cyclic voltammogram.
The object of this invention is to provide a method for producing a compound oxide superconducting thin film having a high critical temperature.